This invention relates to ion exchange materials, and more particularly to those ion exchange materials prepared by functionalizing partially pyrolyzed, macroporous polymer particles containing a carbon-fixing moiety. It further relates to the use of such materials for removal of ions from dense liquids.
Carbonaceous materials, such as charcoal, anthracite, activated carbon and peat have been functionalized in the past to produce compositions with ion exchange activity. Such ion exchange materials suffer from disadvantages common to carbonaceous adsorbents, including lack of control over starting materials, and friability of active carbon particles. They suffer further disadvantages of their own, including a limited number of functionalizing groups, and an interaction of adsorbent and ion exchange properties which makes regeneration difficult. Finally, they tend to have low ion exchange capacities.
As disclosed by Neely in U.S. Pat. No. 4,040,990, which is hereby incorporated into this specification by reference, adsorbent materials have been made by partially pyrolyzing macroporous polymer particles containing a carbon-fixing moiety. These carbonaceous materials possess several unusual properties which set them apart from prior carbonaceous adsorbents, including resistance to particle crushing, elimination of activation processes, and a high degree of control over particle shape and composition. These materials are further distinguished from activated carbon adsorbents by a much lower carbon to hydrogen ratio, being typically between about 2.0:1 and about 10:1, compared with the carbon to hydrogen ratio of activated carbon which is greater than 30:1, and by the presence of carbon-fixing moieties which permit the starting polymer to char without fusing in order to retain the macroporous structure of the polymer particles. The macropores range from about 5 to about 10,000 nanometers in average critical dimension, and are present with at least one other set of pores of a different average size (i.e., multi-modal pore distribution), including smaller pores generally ranging in size from about 0.4 to about 5 nanometers, depending largely upon the maximum temperature during pyrolysis. The carbon-fixing moieties are discussed in the above-referenced U.S. patent of Neely. Although these pyrolyzed polymer particles may be made from polymers containing ion exchange functionality, they are themselves merely adsorbent materials, having lost their ion exchange functionality during pyrolysis. The term, "carbonaceous, macroporous polymer adsorbent particles", shall hereinafter refer specifically to these materials of the Neely patent, containing styrene as one of the ethylenically unsaturated monomers, and pyrolyzed at a temperature from about 300.degree. to about 800.degree. C.
It has been discovered that the carbonaceous, macroporous polymer adsorbent particles disclosed as adsorbents by Neely in the above-referenced U.S. Pat. No. 4,040,990 may be functionalized to prepare novel adsorbents with modified adsorption properties, novel ion exchange materials, or novel precursors thereof. This functionalization may occur subsequent to the pyrolysis described in U.S. Pat. No. 4,040,990, or it may occur simultaneously with such pyrolysis. It is critical to the preparation of the materials of this invention that the copolymers from which these novel materials are derived contain styrene as at least one of the ethylenically unsaturated monomers.
The new functionalized adsorbents, ion exchange materials (which may be of the strong acid, weak acid, strong base or weak base type), and functionalized precursor materials retain the macroporous structure of the original macroporous polymers from which they are derived, but they possess a substantially higher skeletal density, i.e., density of the material excluding void spaces due to particle porosity and to interstitial space between particles in a packed bed. Pore size and distribution may also be different. While these new materials may function as adsorbents because they have the high surface areas characteristic of carbonaceous macroporous polymer adsorbent particles and of macroreticular polymer adsorbent, they also may show enhanced specificity for certain adsorbates, and/or significant ion exchange capacity.
Observed reactions of the carbonaceous macroporous polymer adsorbent particles indicate that they contain many alkyl-substituted polycyclic aromatic hydrocarbon fragments. Examples 1 and 2, below, illustrate spontaneous, room-temperature chlorination of these polymers, which is characteristic of anthracenes, phenanthrenes and higher polycyclic aromatics. Substituted benzenes and naphthalenes do not undergo such a chlorination. Because of this hypothetical structure, it is expected that any conventional electrophilic substitution or addition that proceeds with polycyclic aromatic compounds, and any conventional free-radical substitution reaction that proceeds with aralkyl compounds and polymers such as alkyl-substituted polystyrenes, will be available for functionalizing the partially pyrolyzed carbonaceous macroporous polymer adsorbent particles. The functionalization reactions used are well known, and several are illustrated in the examples.
These functionalization reactions include, but are not limited to, halogenation, halosulfonation, sulfonation, halomethylation, amidomethylation, sulfonation, and oxidation, either alone or followed by amination, or amination and quaternization. The resulting functional groups include precursor groups to the ion exchange groups, such as covalent chlorine, covalent bromine, chloromethyl, bromomethyl and acylaminomethyl, and the ion exchange groups themselves such as sulfonyl chloride, sulfonic acid, amidomethyl, and carboxylic acid. The sulfonic acid functional group provides strongly acidic cation exchange functionality, and the carboxylic acid group provides weakly acidic cation exchange functionality. Anion exchange functionality may also be imparted to carbonaceous macroporous polymer adsorbent particles which have been functionalized with groups listed above. Thus, weakly basic anion exchange functional groups may be produced by (a) reacting chloride, bromide or chloromethyl groups with primary or secondary amines, and preferably those amines having 1-6 carbon atoms, (b) reacting chloride, bromide, chloromethyl, chlorosulfonyl or carboxylic acid groups with polyamines containing at least one primary or secondary nitrogen, and preferably those amines having 1-6 carbon atoms per amine nitrogen, or (c) hydrolyzing acylaminomethyl groups. Strongly basic anion exchange functionality may be produced by alkylating weakly basic groups with an alkylating reagent such as methyl chloride.
The temperature at which the polymers are pyrolyzed has an effect on the number of alkyl-substituted, polycyclic, aromatic hydrocarbon fragments available in the pyrolyzed polymer. At very low pyrolysis temperatures the formation of new carbon-carbon bonds to produce the polycyclic fragments is limited, and at very high pyrolysis temperatures the structure tends to become graphitic. Polymers pyrolyzed at temperatures from about 300.degree. C. to about 800.degree. C. are preferred for functionalization according to this invention.
It should be noted that the carbon-fixing moiety itself does no furnish functionality. For example, the sulfur and oxygen of the samples in Table III, Example 26, below, do not necessarily enter into further functionalization reactions, nor do they themselves contribute any ion exchange functionality. Similarly, chlorine may act as a carbon-fixing moiety in polymers such as vinylidene chloride or vinyl chloride, or when it is present on the polymer backbone of aromatic polymers. Here the chlorine is thought to fix carbon by means of a dehydrohalogenation reaction. The resulting pyrolyzed materials do not contain functional chlorine, although non-functional chlorine may be present at levels as high as 2-3%. The following is a further example of the difference between carbon-fixing moieties and functional groups, as shown in a simultaneous pyrolysis and chlorination performed similarly to the reactions of Example 26, below, but using a polymer which does not contain a carbon-fixing moiety such as the sulfonic acid of Resin A (below). In this example chlorine initially acts as a carbon-fixing moiety, causing the dehydrohalogenation which promotes carbon-fixing, but as the temperature increases, additional chlorine subsequently reacts with the partially pyrolyzed polymer in such a way as to form functional groups. Thus, in a single material some chlorine atoms are acting as carbon-fixing moiety, as described in U.S. Pat. No. 4,040,990, while other chlorine atoms are acting as the functional groups of the present invention. Similarly, functionalized materials of the present invention, especially the halogenated materials, may be re-pyrolyzed subsequent to functionalization. Functional groups such as the functional halogens may remain in the material following such a second pyrolysis. Although carbonaceous macroporous polymer adsorbent particles which otherwise would have no functionality following pyrolysis, but which have functionality as a result of simultaneous or subsequent functionalization, are the materials of the present invention, as a practical matter the preferred materials of the present invention contain a significant level of functionalization. Such preferred materials contain at least about 3%, more preferably at least about 5%, and most preferably at least about 8%, by weight of the functionalized material, of subsequently reactable functional groups, or they contain demonstrable ion exchange activity, preferably at least about 0.2 milliequivalents/gram, more preferably at least about 0.5 milliequivalents/gram, and most preferably at least about 1 milliequivalent/gram, each on a dry basis, of ion exchange material.
The high skeletal densities of the functionalized, carbonaceous macroporous polymer adsorbent particles and their high physical strength--typically significantly higher than that of the polymer particles prior to pyrolysis--make these materials especially well suited for moving-bed use in continuous deionization systems, and for use in treating dense streams such as uranyl salt solutions and sugar syrup process streams. The former streams typically have densities in the range of about 1.03 to 1.10 grams per cubic centimeter, while that of high-fructose corn syrup is typically about 1.18 grams per cubic centimeter. Densities of conventional resins used to treat such streams tend to be very close to these values, so that resin beads tend to float or settle very slowly. This necessitates special handling procedures or equipment to separate the resin beads from the process stream. The ion exchange particles of this invention have significantly higher densities than conventional, styrene-based ion exchange resins because their polymeric frameworks have higher skeletal densities. The skeletal density of Resin A (identified below), pyrolyzed at 500.degree. C. is 1.34 g/cm.sup.3, and that of the same resin pyrolyzed at 800.degree. C. is 1.98 g/cm.sup.3, compared with a skeletal density of 1.07 g/cm.sup.3 for crosslinked polystyrene. These dense particles settle quickly in the relatively dense streams described above, and as a result permit conventional resin-bed techniques such as countercurrent-flow washing to be used in the processes for treating such streams.